To the system disclosed in the applicant's prior U.S. Pat. Nos. 4,126,823 and 4,260,949, applicant herein adds a timer referred to as a delayed quasi-minimum timer or DQM Timer. There takes place a measurement of the time interval from the start of the beat frequency waveform to some subsequent designated beat minimum. The measurement to the first minimum is the most sensitive.
The present invention provides important advantages. For instance, sensitivity is enhanced by making a measurement of the time difference between the moment of occurrence of a no-search-object beat minimum and the different moment of occurrence of the same beat minimum shifted in time by the presence of an object of the search, a technique based upon second differential magnetometry.
The no-search-object beat is effectively infinitely far out in time in prior art systems. That is, the prior art utilized first differential magnetometry. By way of example, the prior art may be taken as contained in an article entitled "A Gradient Magnetometer Using Proton Free-Precession" a journal article by M. J. Aitken and M. S. Tite, Journal of Scientific Instruments, Vol. 39, p. 625 (1962).
A necessary but not sufficient condition for performing second differential measurements on prior art circuits is the introduction of a constant beat frequency as their no-search-object signal. However, prior art devices cannot cope with a catastrophic limitation, namely the lack of reproducibility of peak precession voltages from one polarization phase to the next. Thus, reproducibility on the part of prior art systems is widely considered to present a difficult problem, even if the prior art had recognized the value of and had introduced a constant beat frequency as the no-search-object signal.
The present invention is directed to improving sensitivity and capabilities of a second differential proton magnetometer, the specific type of differential magnetometer associated with applicant's previous patents.